1. Field of the Invention
The invention relates to a field-effect semiconductor device evaluation apparatus and a field-effect semiconductor device evaluation method.
2. Description of the Related Art
Reduction in thickness of an SiO2 film used as a gate insulating film has advanced with the advance of fineness in MISFET (Metal-Insulator-Semiconductor Field Effect Transistor). For example, it is considered that an SiO2 film having a thickness of 1 nm or less will be necessary for an MISFET of a generation, which will have a gate length smaller than 50 nm.
In such a thin SiO2 film, there is however a problem that leakage current through the gate insulating film increases. As a measure against this problem, an attempt has been made to use a substance having a high dielectric constant compared with SiO2, for the gate insulating film.
It has been known here that carrier mobility of the MISFET in the case where such a high dielectric constant gate insulating film (so-called high-k insulating film) is applied to an actual device is different from that in the case of SiO2. It is therefore necessary to design a highly integrated circuit appropriately after exactly grasping the carrier mobility of the MISFET, which varies according to the high dielectric constant material, in order to use the high-k insulating film efficiently in the highly integrated circuit.
It is however difficult to evaluate the mobility of a high-k MISFET. Generally, a large number of levels capable of trapping electric charge are in the high-k insulating film unlike the case of SiO2. In a conventional evaluation method (Split-CV method), electrification is caused by electric charge trapped by levels in the film when the gate voltage is applied. Because the electrification exerts a bad influence on analysis, the mobility cannot be estimated correctly.
There have been recently proposed several methods in which the time of application of the gate voltage is shortened (the gate voltage is input as a pulse voltage) to prevent levels in the high-k insulating film from trapping electric charge to thereby estimate the mobility accurately (see Non-Patent Documents 1 and 2).
[Non-Patent Document 1] A. Kerber et al. “Direct Measurement of the Inversion Charge in MOSFETS: Application to Mobility Extraction in Alternative Gate Dielectrics”, Symp. On VLSI Tech., p. 159 (2003)
[Non-Patent Document 2] D. V. Singh et al. “Ultra-fast Measurements of the Inversion Charge in MOSFETs and Impact on Measured Mobility in High-k MOSFETs”, Tech. Dig. of IEDM, p. 863 (2004)
Each of these evaluation methods, however, has any one of the following problems.
In the evaluation method represented by Non-Patent Document 1, it is necessary to apply “continuous” pulses for measurement.
If such continuous pulses are used, trapping of electric charge is caused by application of a gate voltage even though the gate voltage is a pulse voltage. This problem arises more remarkably as the frequency becomes higher. If a long time is set for application of the continuous pulses to obtain high accuracy in measurement, characteristic of the MISFET to be evaluated changes or crashes. It is hence difficult to calculate mobility accurately.
In the evaluation method represented by Non-Patent Document 2, respective measuring systems for measuring inversion layer carrier density Ns and inversion layer sheet resistance ρch necessary for deduction of mobility are different from each other.
When the two physical quantities Ns and ρch are measured by different measuring systems, strict correspondence between the two physical quantities is spoiled so that variations according to measurement conditions become large. As a result, accurate mobility cannot be estimated. Moreover, when measurement is shifted from one measuring system to the other measuring system, a great deal of time and labor is required.
Specifically, at the time of measuring ρch, the source terminal is electrically connected to the ground. Accordingly, the electric potential of the inversion layer formed in a surface of the MISFET is unchanged because the electric potential of the source region is always fixed at a ground level. On the other hand, at the time of measuring Ns, the source and drain terminals are electrically connected to each other and electrically connected to the ground through a capacitor. Accordingly, the electric potential of the inversion layer formed in the surface of the MISFET varies during measurement. As a result, the potential environments for the inversion layer at the times of measuring the two physical quantities are different from each other.
It is therefore necessary to construct an evaluation method in which the two problems can be solved at once to estimate mobility of the high-k MISFET accurately.